Harry Jacobs, a fast-talking lawyer from New Jersey who may prove to be Al Gore's last legal lifeline, will go to court today to back the vice-president's claim that Republicans tampered illegally with thousands of ballots in Florida's Seminole county.

Mr Jacobs is suing the elections supervisor, Sandra Goard, and calling for all 15,000 of the absentee ballots cast across the county to be thrown out. As George W Bush won the absentee vote in Seminole by a clear majority, the rejection of the ballots would overwhelm the Texas governor's 537-vote lead in Florida and hand Mr Gore victory in the state, and therefore the presidency.

Mr Bush's supporters have portrayed Mr Jacobs as a Democratic pawn who is simply doing the bidding of the Gore campaign, and Republican lawyers are due in court in Tallahassee today to try to have his case thrown out as without merit, before a full hearing tomorrow.

Mr Jacobs insists that he is acting out of outrage after overhearing a remark by a Seminole county election official. The official said the local Republican elections supervisor had allowed Republican officials to fill in voter identification numbers on incomplete absentee ballot forms being sent out to their supporters.

Republicans say the controversy has been confected from a simple printing error. Before the elections, both parties send absentee ballot applications to their supporters living abroad or out of their home state. More than 2,000 Republican forms, however, came out of the printer without the any voter identification number.

In response to a Republican request to have the forms completed, Ms Goard allowed two party workers to fill in the missing numbers in her office.

Local Democrats are suing Martin county, just north of Miami, where another Republican elections supervisor, Peggy Robbins, allowed Republican officials to remove faulty ballot application forms from her office, fill in missing information and return them to her a few days later. That case is also due to be heard in Tallahassee tomorrow.

The Gore campaign has distanced itself from the Jacobs case, almost certainly because, if successful, it would lead to the invalidation of absentee votes from members of the armed services posted abroad, a controversial move. But the case may prove to be the vice-president's last hope if he loses his formal contest of the Florida election result.

Republican officials point to the fact that Mr Jacobs has contributed to the Gore campaign and that he has admitted that he had sought advice about his lawsuit from a colleague, Mitchell Berger, who is also a Gore fundraiser in Florida. The conversation, Republicans argue, proves Mr Jacobs is surreptitiously taking instructions from the Gore campaign.

Mr Jacobs' lawyer, Gerald Richman, rejected the claims. "They're absolutely, totally false," Mr Richman said yesterday. "He just called a lawyer he knew to seek advice. The Republicans just want to avoid the merits of the case."

Lawyers for the Bush campaign have tried to oblige Mr Gore to appear as a witness. They have also called on the judge hearing the case, Nikki Clark, to withdraw, on the grounds that she has a vested interest. She was recently overlooked by Florida's governor, Jeb Bush (George W's brother) for a job in the state appellate court.